1. Field of the Invention
This invention pertains to semiconductor packaging in general, and in particular, to an improved method for molding the plastic bodies of semiconductor packages.
2. Description of the Related Art
Integrated circuits ("ICs") are formed on a single die, or "chip," cut from a semiconductor wafer containing a large number of identical dies. The dies are relatively small and fragile, are susceptible to harmful environmental elements, particularly moisture, and generate a relatively large amount of heat in a relatively small volume during operation. Accordingly, ICs must be packaged in affordable, yet robust, packages that protect them from the environment, permit them to be reliably mounted to and interconnected with, for example, a printed circuit board ("PCB") populated with associated electronic components, and to dissipate effectively the heat they generate during operation.
In both leadframe and grid array types of packages, a leadframe or substrate assembly having a plurality of leads or traces and a semiconductor die attached to it and electrically connected to the leads is placed in the cavity of clam-shell molding tool, and a molten plastic, typically, an epoxy resin, is transferred into the cavity of the mold to form a hard, strong body over the die, interconnects, and leads or traces, to seal and protect those components from the environment.
A problem with the conventional package transfer molding technique relates to the tendency of the molding compound to adhere to the surfaces of the molding tool, including the cavity, chase and ejector pins of the mold, that come into contact with the plastic molding compound during the molding process. If the molding compound adheres appreciably to these surfaces of the mold, the package will not release completely from the mold, resulting in the formation of cracks or craters in the package body, defective packages, and/or the subsequent penetration of the package by damaging moisture.
Another, countervailing, package molding problem relates to an incomplete or defective adhesion of the plastic package body to the package components, i.e., the leadframe or substrate, wire bonds, the die, and other components. This can result in a delamination of the plastic body from the components and the subsequent propagation of cracks and penetration of the package by moisture.
In particular, where delamination occurs at a boundary of the package body, a microscopic crack is created for the penetration of the package by moisture. This penetration can wreak a two-fold assault on the package: First, the moisture can corrode any metallizations present in its path, resulting in subsequent current leakage through the corrosive path; and second, the moisture can expand and contract with temperature cycling of the package, resulting in further propagation of the cracks into the package, and hence, further penetration of the package by moisture.
To address the mold-sticking problem, a "mold release agent" is usually incorporated into the molding compound. However, this can create an additional problem, in that, while the mold release agent prevents the molding compound from adhering to the molding tool, it also reduces the adhesion of the molding compound to the components of the package. To offset this reduction, an "adhesion promoter," or "coupling agent," is also typically added to the molding compound to enhance adhesion of the compound to the components of the package.
However, this latter addition can also create some problems, in that coupling agents can increase the melt viscosity of the molding compound, thereby reducing the surface wetting of the lead frame surface and other components of the package by the molding compound. This increase in viscosity is caused by the premature cross-linking of the base resin and the hardener during the transfer of the molding compound into the mold cavity due to the relative overabundance of the coupling agent. This reduction in surface wetting can result in imperfect adhesion between the molding compound and the components of the semiconductor package, and hence, voids, cracks, and potential delamination.
It is therefore desirable to provide an improved method of molding a plastic body onto a semiconductor package that reduces or prevents adhesion between the body and the molding tool, thereby preventing the molding of defective package bodies, and permitting the amount of both release agents and adhesion promoters used in the molding compound to be substantially reduced, or eliminated altogether. This would result in a stronger semiconductor package body having improved adhesion with the components of the package, and hence, an improved resistance to the propagation of cracks and subsequent penetration of the package by moisture.